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Solar Power – It is often said that solar power plants can’t ‘do baseload’ – that they only provide power when the sun shines. Not so: Richard Keech and Matthew Wright* explain how solar thermal plants work and how the Spanish now get solar power around the clock using advances in thermal storage. T he talk about climate change has seen a renewed interest in power. After all, electricity generation is the largest single man-made source of greenhouse gas emissions, mostly from the burning of fossil fuels, usually coal or gas. But sustainable alternatives exist – one of the most exciting is Concentrating Solar Thermal (CST) power [sometimes called Concentrating Solar Power (CSP)]. Traditionally, solar electricity generation has been only when the sun shines. Hence the great appeal of new CST plants which can store their heat and generate power even at night. Solar resource At ground level, the power of the Sun on a one meter square surface, at right angles to the Sun’s rays is about 1kW. Excluding cloud effects, this gives an average of about 6kWh/day for every square meter in sunlight. If you do the numbers this represents a phenomenally large resource. Australia’s total current electrical peak generation capacity (about 49GW) is equivalent to what falls as sunlight on an area of about 8km x 8km (at noon at Southern Australian latitudes) or about 0.001% of the Australian landmass. When you take into account typical sunlight patterns, typical plant efficiency and layout, you would still need less than 0.05% of Australia’s land area to generate equivalent power. To put the required land area in perspective, it would fit six times into Anna Creek, Australia’s largest cattle station. It is clear that in a country like Australia, the solar resource greatly exceeds our energy needs. CST technologies Before considering how solar plants can run at night, let’s review the underlying technology of CST. They have in common the basic principle * Executive Director, Beyond Zero Emissions 10  Silicon Chip siliconchip.com.au – 24/7 of capturing solar energy to heat water to generate steam (see box below‘not all steamed up’). This steam powers a turbine, which in turn spins an electric generator to create AC power. From the point at which the steam is generated, a CST plant is similar to coal, gas or nuclear in its operating principle. A solar plant is distinguished by how that steam is generated in the first place. To capture solar energy, mirrors reflect the sun’s rays to a central collection point. Different arrangements of mirrors exist. Broadly speaking these are: troughs, power towers, linear fresnel and dishes. Trough technology In a trough configuration, long lines of mirrors with a parabolic cross section focus solar radiation on a pipe. A fluid pumped through the pipe to pick up the solar energy is heated to around 400°C. The fluid is usually a high-grade synthetic oil which does not boil or degrade at high temperatures. These oils are only suitable up to about 400°C. Trough mirro rs from a Span solar power plant (BZE ph ish oto). Trough technology is the most proven CST design. The largest solar generation facility in the world, SEGS (a set of nine plants near Kramer Junction in the Mojave desert California), uses troughs. Jointly they have a capacity of 354MW. In a trough plant, the mirrors rotate around their long (North-South) axis to track the Sun during the day. Because they remain horizontal and so don’t track the Sun’s elevation, trough mirrors are most effective close to the equator. At the latitudes of Southern Australia, trough mirrors are only about half as effective as a mirror that can track the sun. This is due to the projection effect (see ‘capturing the sun efficiently’). Linear Fresnel The curved mirror structures of a trough plant are very expensive. A less-expensive variant on the trough mirror configuration is a Linear Fresnel (pronounced ‘frenell’). These systems use long, near-flat mirrors close to the ground to make an optical approximation of a parabolic trough, without the structural complexity. Not all steame d up The main tech nology used in is steam turb present-day el ines. However ec it’s worth notin trical generation power plants are being built g that some solar which don’t us Some are us e steam. in Stirling heat en g concentrating photovol taic and som gines. Neither e use of these techno able to provid e effective en logies are curr ergy storage. ently siliconchip.com.au August ugust 2010  11 can be more efficient and cheaper than that from a trough configuration which heats to about 400°C. The turbines required in conjunction with a tower are the same as those used in coal-fired plants, whereas the turbine technology required for lowertemperature operation is considerably more expensive because of the much lower economies of scale. Heat Storage A Linear Fresnel assembly (from Areva Solar – formerly Ausra. (http://en.wikipedia.org/wiki/File:Fresnel_reflectors_ausra.jpg) The systems from Biotec Novasol (owned by Australian company Transfield) and Areva Solar (formerly Australian company Ausra) both have relatively low operating temperatures (of around 290°C) and therefore there is no viable commercially available storage method. Linear Fresnel companies are moving to higher temperatures and pressures. Mann Ferrestel/Solar Power Group are offering 450°C in a Linear Fresnel configuration. Dish technology Mirrors in a dish configuration are effective at concentrating the sun and track the sun in two axes. Previously they’ve been expensive and not often used in production solar energy plants. Australia’s first solar thermal power plant was a dish-based facility at White Cliffs in NSW which operated from 1981 to 1996. This was a 25kW plant for an off-grid community and was developed by the ANU. The ANU has also developed the world’s biggest mass production solar dish system. The ANU SG4, a fourthgeneration dish is now ready for mass production. Their innovative manufacturing system involves a ‘factory in the field’. It is built in the field on a very accurate jig, instead of adjusting the dish after it has been manufactured. field of near-flat, independently controlled mirrors called heliostats to focus sunlight on a central receiver at the top of a tower. Tower configurations can scale up to configurations involving many hundreds or even thousands of mirrors. This gives towers the greatest capacity to concentrate the sun’s rays, leading to higher operating temperatures. Heliostats are spaced to ensure they don’t overshadow each other. A modern tower-based solar plant would typically pass a fluid through the receiver to be heated up to about 570°C (and in future up to about 650°C). At this temperature, electrical generation In November 2008 the 50MW Andasol 1 CST plant near Granada in southern Spain started feeding power to the grid. What was most interesting about this plant was its ability to supply power to the grid around the clock using a system of heat storage in tanks of molten salt. The adjacent Andasol 2 plant has since come online and doubled the capacity to 100MW. Andasol 3 is under construction now, with Andasol 4 in planning. These CST plants are each rated as having 7.5 hours of thermal storage. This number represents the storage when running at the full rated output of the plant. Operators can choose to run at lower output for longer periods, giving the plant round-the-clock generating potential. Molten salt The use of molten salt as a storage medium has been proven for some time (the French had a prototype test plant in the early 1980s). The US Department of Energy had a commercial Tower systems Tower-based systems use a large 12  Silicon Chip An SG4 dish mirror from ANU in Canberra (BZE photo) siliconchip.com.au Capturing the sun efficiently Heliostat mirrors track the sun in two axes which makes them more efficient than horizontal trough mirrors, especially in winter and when sited further from the equator. Compared with a dish, which gives the best sun tracking, a trough mirror captures about 75% less energy in winter at temperate latitudes because of the low angle of the sun. This reduction of collection capacity is called the projection effect. Solar engineers use the term insolation to describe the measurement of received solar energy, and Direct Normal Incidence (DNI) to describe the solar energy available to collectors which track the sun, ie, no projection effect. For horizontally-configured mirrors the insolation is less (due to the projection effect) and measured as Global Horizontal Irradiance (GHI). DNI and GHI are often confused and this confusion can cause illinformed assessments suggesting low performance for solar thermal The projection effect comparing vertical sun’s rays with rays at 30°. systems, when in fact with systems that use direct beam radiation without significant projection effect (dish and tower) perform very well in the right climatic zones all year round even at higher latitudes. The PS10 power tower near Seville in Spain (BZE picture) demonstration plant called Solar Two operating with storage in the 1990s. The salt mixture generally used is 40% Potassium Nitrate and 60% Sodium Nitrate. The salt, which is chemically very close to fertilizer, has a number of properties that make it suitable: • It is stable as a liquid over a large temperature range • It is reasonably priced • It is non-corrosive • It can be used in unpressurised, insulated carbon steel vessels. In operation, salt is pumped between two large tanks – one hot and the other (notionally) cold. The salt mixture has a freezing point of about 225°C (depending on formulation) and needs to be kept in liquid form at all times. So the ‘cold’ tank is operated at about 285°C, while the hot tank can hold a temperature of 400°C or higher. There is no material phase change involved in the use of the salt. Andasol is a trough plant. When its mirrors are collecting sunlight, it heats a synthetic oil. In turn this is passed siliconchip.com.au through a heat exchanger to re-heat the ‘cold’ salt which is pumped back into the hot tank. When electrical generation is required, the liquid salt is pumped through a steam generator to drive a conventional Rankine-cycle steam turbine and then into the ‘cold’ tank. Andasol uses Siemens SST-700 turbines, which are widely used in the power industry (www.energy.siemens. com/hq/en/power-generation/steamturbines/sst-700.htm). Power towers and storage Since Andasol was commissioned, attention is on the Gemasolar (pronounced ‘hemasolar’) Solar Tres project near Seville in Southern Spain. Solar Tres, currently under con- Molten salt tanks at Andasol (picture: BZE) August 2010  13 of cloud, it will be necessary to have some form of backup energy source. Future solar plants could, for example, utilise a renewable, low-value biofuel. Once backup is incorporated, a solar plant could provide the year-round dependability required to underpin a modern energy economy. Backup is of most benefit when a plant is installed on an isolated grid. However, on larger grids and grids integrated with wind power, zero or very minimal co-firing with biomass, storage hydro or pumped storage hydro would be required. Solar Thermal in the US The rotor of a Siemens SST-700 turbine (Photo: Siemens). struction, is a power tower rated at 17MW with 15 hours storage. When commissioned it will be the first commercial power tower with storage and will take advantage of the higher operating efficiencies possible using this configuration. The salt will be heated to about 565°C. At these temperatures, each MWh of energy generated requires about 25 tonnes of salt. Plants operating at lower temperatures require proportionally more salt per unit of energy stored; with an upper temperature of 400°C, the energy stored is about 1MWh per 75 tonnes of salt. Changing the relative sizing of the mirrors, storage and turbine allows for different balance between maximum power and energy storage. In the case of Solar Tres, with 15 hours of storage at full power, gives true baseload capacity. The trade off between power and storage is shown below. These levels of average utilisation (about 75%) compare favourably with Australian baseload coal-fired power plants. On average NSW coal plants operate at an average 63% of rated capacity. Bad weather backup To deal with the inevitable periods Spain has taken the early lead with the first commercial CST-with-storage plants. However in the USA, the Bureau of Land Management (BLM) has received over 100,000MW of plant approval applications on BLM land in six states alone. Half of all these applications are for tower-type systems with much of those using molten salt as a working fluid and 24-hour dispatch storage media. This year, according to the head of the SEIA, Fred Morse and head of Sandia National Laboratories, Thomas Mancini, it is expected that around 11 large scale solar thermal plants will be started. Each of these projects will take less than two years to build. Contrast this with 12-24 months for wind plants, 5-7 years for coal plants and 7.5-19 years for nuclear plants. Costs Currently first-of-a-kind, mediumscale solar plants with storage are being built in the US which can generate power at about 20 cents per kWh. New technologies always follow a cost-reduction curve once they mature and economies of scale are realised. Research suggests that when the installed capacity expands by a further 8700MW, CST plants could reach cost parity with new coal and gas plants, providing power at around 5c per kWh. Re-powering Australia Trading of power and storage (source: BZE Solar Flagships). 14  Silicon Chip Australia’s entire energy needs could conceivably be met with a 60:40 mix of Spanish-style solar thermal and wind. Other technologies, such as geothermal and wave power, show promise siliconchip.com.au ANTRIM TRANSFORMERS manufactured in Australia by Harbuch Electronics Pty Ltd harbuch<at>optusnet.com.au Toroidal – Conventional Transformers Power – Audio – Valve – ‘Specials’ Medical – Isolated – Stepup/down Encased Power Supplies Toroidal General Construction OUTER INSULATION OUTER WINDING WINDING INSULATION INNER WINDING CORE CORE INSULATION Comprehensive data available: www.harbuch.com.au Harbuch Electronics Pty Ltd 9/40 Leighton Pl, HORNSBY 2077 Ph (02) 9476 5854 Fax (02) 9476 3231 The Gemasolar Solar Tres plant under construction in February 2010 (photo: Torresol Energy). but solar thermal and wind can be deployed at scale today and could be sufficient to entirely power the Australian electricity grid. This research has been done as part of the Zero Carbon Australia 2020 plan at www.zerocarbonplan.org Conclusion The Spanish are really onto something. Using technology pioneered by the French and Americans, they have demolished the myth that you can’t do baseload power with renewable energy. siliconchip.com.au There are now two companies, Torresol from Spain and Solar Reserve from the USA, with commercially available solar power system with storage. These systems’ operating characteristics compare well with conventional coal, nuclear or gas combined cycle plant. We no longer have to wait for years of research to bear fruit and we no longer have any excuse to delay. The future of renewable energy seems bright indeed. SC August 2010  15